This section provides background information related to the present disclosure which is not necessarily prior art. This section also provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Micro ramps have been proposed as passive boundary layer control features to reduce the boundary layer thickening and separation that are typical in a wide range of boundary layer control applications. Such features can produce vortices whose axes of rotation are aligned with the streamwise direction and pull high energy fluid from the free stream deep into the boundary layer to avoid separation and other unwanted effects. The more energetic boundary layer that results has a reduced shape factor and is less susceptible to thickening and separation produced by an adverse pressure gradient.
The standard micro ramp design, generally referred to at 100, was introduced by Anderson et al. and consists of triangular features as shown in FIG. 1. The flow, U, is initially deflected by the micro ramp 100 before it passes over angled dumps 102 on each side of the feature. As seen in FIG. 2, a counter-rotating vortex pair 104a, 104b is produced, the size and strength of which depends on the height of the feature h (FIG. 1) and the half-angle of the triangle Ap.
Still referring to FIG. 2, these counter-rotating vortices 104a, 104b are illustrated from a downstream location, looking in the upstream direction at the rearward faces of the micro ramp 100. Of critical importance is the behavior of these streamwise vortices 104a, 104b as they propagate downstream. Note that the effect, F104b, of vortex 104b on vortex 104a is to push vortex 104a in the upward direction, and that the same is true for the effect, F104a, of vortex 104a on vortex 104b. The effect of the wall 106 on the vortex pair 104a, 104b will be the same as the effect from an “image” vortex pair, which is a reflection of the real vortices across the wall. The image vortex pair is shown by the dotted lines in FIG. 2. It can then be seen that the effect of the wall 106 is to pull the vortex pair toward each other, which has the detrimental effect of causing the opposite-signed vorticity in the streamwise vortices 104a, 104b to interdiffuse and cancel, thereby reducing the vortex strengths as they propagate downstream.
Thus the effect of the vortex-vortex interaction in a conventional design is to drive the vortex pair 104a, 104b up out of the boundary layer, and the effect of the wall 106 is to pull the vortex pair together. Since the initially separate vortices are counter-rotating, the strength of vortex 104a cancels the strength of vortex 104b as the pair is drawn together. As the vortex pair propagates downstream, then, they are located in an undesirable position above or within the boundary layer with their strength diminished. It would be desirable for exactly the opposite interactions to take place as the vortices move downstream; the vortex pair should remain separated, remain close to the wall 106, and preferably remain within the boundary layer.
In other words, each individual micro ramp 100 creates a pair of streamwise vortices 104a, 104b that cause the resulting vortex pair to naturally lift upward as a consequence of the mutual Biot-Savart induced interaction between the two vortices. This in turn causes the vortices to quickly lift out of the boundary layer, reducing their efficacy in controlling the boundary layer. Moreover, the effect of the wall on the resulting streamwise vortices is a further Biot-Savart induced interaction with their image vortices that causes the vortices 104a, 104b to move toward each other and thereby reduce their strengths through interdiffusion, thus further reducing their efficacy in controlling the boundary layer. It is both these aspects of such prior art embedded passive boundary layer control devices that limit their efficacy, and that the disclosed new devices are able to overcome to achieve substantially greater efficacy.
According to the principles of the present teachings, in some embodiments, ramp-like vortex generators are provided that can be mounted on any surface such as the wing of an airplane, the inlet to a propulsion system, the hull of a ship, or any other surface over which a fluid moves, when the objective is to minimize drag, irreversibility losses, or other performance penalties that can occur for reasons such as boundary layer separation or other undesirable boundary layer properties. The disclosed devices act to passively induce streamwise vortices in the boundary layer, thereby transferring high-momentum fluid toward the surface in such a way as to alter the shape of the velocity profile within the boundary layer and thereby avoid or delay separation or alter other properties of the boundary layer. Moreover, the disclosed devices can achieve these beneficial effects with lower performance penalties than prior art devices can.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.